Add note about fileno not being usable as a normal file descriptor in Windows.
diff --git a/Doc/lib/libsocket.tex b/Doc/lib/libsocket.tex
index 199b97f..de2f51d 100644
--- a/Doc/lib/libsocket.tex
+++ b/Doc/lib/libsocket.tex
@@ -480,6 +480,10 @@
\begin{methoddesc}[socket]{fileno}{}
Return the socket's file descriptor (a small integer). This is useful
with \function{select.select()}.
+
+Under Windows the small integer returned by this method cannot be used where
+a file descriptor can be used (such as \function{os.fdopen()}). \UNIX{} does
+not have this limitation.
\end{methoddesc}
\begin{methoddesc}[socket]{getpeername}{}